The present invention relates to a method of and an apparatus for controlling an air-fuel ratio and an ignition timing in an internal combustion engine. More particularly, the invention is concerned with a method of and an apparatus for controlling air-fuel ratio and an ignition timing in an internal combustion engine, wherein the mode of control of the air-fuel ratio is switched in accordance with the state of the engine operation between a feedback control for maintaining the air-fuel ratio substantially at the same level as the stoichiometric level or an open loop control in which no feedback is conducted and a lean control for maintaining the air-fuel ratio at the leaner side of the stoichiometric level, and wherein the ignition is conducted at different ignition timings matching for respective control modes.
Generally, in automotive engines equipped with an exhaust gas scrubber of ternary catalyst type, it is necessary to effect the air-fuel ratio control such that the air-fuel ratio, which is directly related to the condition of combustion in the engine, is always maintained around the stoichiometric level, in order to keep the exhaust emissions clean.
To cope with this demand, a feedback control method has been proposed in which the oxygen content in the exhaust gases is detected by an O.sub.2 sensor as an index of the air-fuel ratio of the mixture, and the air-fuel ratio control is conducted in accordance with the output from the O.sub.2 sensor such that the air-fuel ratio coincides with the stoichiometric ratio. When the engine is operating under comparatively light load, it is possible to decrease the rate of fuel consumption by maintaining the air-fuel ratio at the leaner side of the stoichiometric value without being accompanied by substantial degradation of the exhaust emissions because, under the light load, the rate of generation of nitrogen oxides is sufficiently small.
Under these circumstances, an automotive engine has been proposed in which the control operation mode is selectively switched between the feedback control mode for maintaining the air-fuel ratio substantially at the stoichiometric level and the lean control mode for maintaining the mixture at the leaner side of the stoichiometric level through a feed-forward control, thereby to decrease the rate of fuel consumption.
In the operation of the engine employing the air-fuel ratio control explained above, the ignition timing in the lean control mode is advanced as compared with that in the feedback control mode. The degree of advance angle for the ignition timing is greater as the air-fuel ratio becomes greater. When the control mode is changed from the lean control mode to the feedback control mode or vice versa, the ignition timing is controlled simultaneously with the switching of the control mode.
If the engine has a comparatively large length of the intake passage leading from the fuel injector to the combustion chamber, it takes a substantial time for the fuel injected from the fuel injector to reach the combustion chamber. Therefore, when the control mode is switched from the lean control mode to the feedback control mode depending on the state of the engine operation, an ignition plug is ignited at the retarded advance angle optimum for feedback control mode upon switching of the control mode, while the feedback-controlled fuel reaches the combustion chamber after the change of the ignition timing. Therefore, the inconveniences such as a decrease of the torque and degradation of the response characteristics of the engine take place in the transient period until the effect of the feedback control becomes appreciable. To the contrary, when the control mode is changed from the feedback control to the lean control, the ignition plug is ignited at the advanced advance angle optimum for lean control mode upon switching the control mode, while the decreased fuel by the lean control mode reaches the combustion chamber after the change of the ingition timing. Therefore, knocking may occur in the engine in the transient period until the effect of the lean control becomes appreciable.
These problems are encountered also in the air-fuel ratio controller in which the control mode is switched in accordance with the state of the engine operation between the lean control mode and an open loop control mode in which the fuel is supplied to achieve a stoichiometric air-fuel ratio in accordance with the engine speed and the load level without detecting the air-fuel ratio.